For a spark plug, not only a longer life for realizing maintenance-free but also an improvement in ignition performance and an improvement in combustion efficiency by reducing a size of electrode have been demanded. In order to satisfy these demands, a spark plug including a noble metal tip of platinum, iridium, or the like joined to a spark discharge portion of a center electrode has been frequently used. To further improve the ignition performance, it has been proposed that noble metal tips are joined not only to the center electrode but also to a ground electrode (outer electrode) (refer to Patent Document 1).
Patent Document 1: JP-A-11-121142
When such a noble metal tip is joined, in order to prevent a removal of the noble metal tip from the electrode base member, a method is used in which, for example, on the center electrode, the noble metal tip is placed on the front end of the center electrode base member and the boundary between the noble metal tip and the front end of the center electrode base member is directly melted by a laser, etc. By thus melting, a fused portion with a comparatively large volume is formed at a joint portion between the noble metal tip and the center electrode base member. This fused portion is made of an alloy containing the component of the noble metal tip and the component of the center electrode base member (for example, a nickel-based material). As a result, the fused portion has a thermal expansion coefficient which is intermediate between the thermal expansion coefficient of the noble metal tip and the thermal expansion coefficient of the center electrode base member, and functions as a stress relaxing layer with a comparatively large volume. Therefore, as aforementioned, the effectiveness in preventing the removal increases.